Dosage compensation is the process that equalizes the amounts of products produced by the single X chromosome in males and the two X chromosomes in females. In Drosophila dosage compensation is achieved by doubling the transcription rate of the male's X chromosome. The genes immediately responsible for the increased transcription rate of the male's X are the four male-specific lethal (msl) genes. We have cloned and characterized the mle gene. The protein (MLE) encoded by mle is produced in both males and females but binds to hundreds of sites along the X chromosome only in males. Since cis-acting X chromosome sequences necessary for dosage compensation are known to be widely scattered along the X chromosome our finding suggests that MLE directly participates in dosage compensation. We have recently cloned a second one of the msl genes (msl-3) and are cloning a third one of these genes (msl-2). We plan to continue our molecular genetic characterization of the genes that comprise the hierarchy controlling dosage compensation with three major aims. These are to: (1) understand the mechanism by which the products of these four genes control the transcriptional level of essentially all genes on the X chromosome to bring about dosage compensation; (2) elucidate the mechanism by which the initial decision to develop as male or female results in the male-specific functioning of the msl gene products; and (3) determine the regulatory relationships, if any, between the msl genes.